No abstract

A molecular triad assembly consisting of an electron donor, a bis(phenanthroline)copper(I) chromophore, and an electron acceptor has been prepared. Under visible-light excitation, this assembly undergoes efficient (ca. 50%) photoinduced, multistep formation of a diradical cation charge-separated state that has a lifetime of >100 ns and stores >1.0 eV of energy. This system constitutes an earth-abundant functional analogue of related Ru(bpy)(3) triad systems.

We report evidence of excited-state ion pair reorganisation in a cationic iridium (III) photoredox catalyst in 1,4-dioxane. Microwave-frequency dielectric-loss measurements combined with accurate calculations of dipolar relaxation time allow us to assign both ground and excited-state molecular dipole moments in solution and determine the polarizability volume in the excitedstate. These measurements show significant changes in ground-state dipole moment between [Ir[dF(CF 3 )ppy] 2 (dtbpy)]PF 6 (10.74 Debye) and [Ir[dF(CF 3 )ppy] 2 (dtbpy)]BAr F 4 (4.86 Debye). Photoexcitation of each complex results in population of highly mixed ligand centered and metal-to-ligand charge transfer states with enormous polarizability. Relaxation to the lowest lying excited-state leads to a negative change in the dipole moment for [Ir[dF(CF 3 )ppy] 2 (dtbpy)]PF 6 , and a positive change in dipole moment for [Ir[dF(CF 3 )ppy] 2 (dtbpy)]BAr F 4 . These observations are consistent with a sub-nanosecond reorganization with the PF − 6 counter-ion, which cancels the dipole moment of the lowest lying excited-state, a process which is absent for the BAr F− 4 counter-ion. Taken together, these observations suggest contact-ion pair formation between the cationic metal complex and the PF − 6 anion and, at most, solvent-separated pairing with BAr F− 4 . The dynamic ion pair reorganisation we observe with the PF − 6 counter-ion may substantially modify both the thermodynamic potential available for electron transfer and kinetically inhibit oxidative catalysis, as the anion moves to cover the positively charged end of the molecule, providing a possible mechanistic explanation for recently observed trends in the catalytic activity of these complexes as a function of anion identity in low-polarity solvents. These tunable ion-pair dynamics could prove to be a valuable tool for tailoring the reactivity of both new and extant photocatalysts.

A series of metal-organic chromophores containing Ru(II) or Ir(III) were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long-lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution-based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal-to-ligand charge-transfer (MLCT) excited state quenching upon addition of TNT and 2,4-dinitrotoluene (2,4-DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink-jet printing and solid-state titrations was used for the solid-state detection of TNT; achieving a limit-of-quantitation (LOQ) as low as 5.6 ng cm(-2). The combined effect of a long-lived excited state and an energetically favorable driving force for the PET process makes the Ru(II) and Ir(III) MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high-energy compounds.

One contribution of 11 to a discussion meeting issue 'Organic semiconductor spintronics: utilizing triplet excitons in organic electronics' . The near-visible-to-blue singlet fluorescence of anthracene sensitized by a ruthenium chromophore with a long-lived triplet-excited state, [Ru(5-pyrenyl-1,10-phenanthroline) 3 ](PF 6 ) 2 , in acetonitrile was investigated. Low intensity non-coherent green light was used to selectively excite the sensitizer in the presence of micromolar concentrations of anthracene generating anti-Stokes, singlet fluorescence in the latter, even with incident power densities below 500 µW cm −2 . The resultant data are consistent with photon upconversion proceeding from sensitized triplet-triplet annihilation (TTA) of the anthracene acceptor molecules, confirmed through transient absorption spectroscopy as well as static and dynamic photoluminescence experiments. Additionally, quadratic-to-linear incident power regimes for the upconversion process were identified for this composition under monochromatic 488 nm excitation, consistent with a sensitized TTA mechanism ultimately producing the anti-Stokes emission characteristic of anthracene singlet fluorescence.

We report evidence of excited-state ion pair reorganisation in a cationic iridium (III) photoredox catalyst in 1,4-dioxane. Microwave-frequency dielectric-loss measurements combined with accurate calculations of dipolar relaxation time allow us to assign both ground and excited-state molecular dipole moments in solution and determine the polarizability volume in the excitedstate. These measurements show significant changes in ground-state dipole moment between [Ir[dF(CF 3 )ppy] 2 (dtbpy)]PF 6 (10.74 Debye) and [Ir[dF(CF 3 )ppy] 2 (dtbpy)]BAr F 4 (4.86 Debye). Photoexcitation of each complex results in population of highly mixed ligand centered and metal-to-ligand charge transfer states with enormous polarizability. Relaxation to the lowest lying excited-state leads to a negative change in the dipole moment for [Ir[dF(CF 3 )ppy] 2 (dtbpy)]PF 6 , and a positive change in dipole moment for [Ir[dF(CF 3 )ppy] 2 (dtbpy)]BAr F 4 . These observations are consistent with a sub-nanosecond reorganization with the PF − 6 counter-ion, which cancels the dipole moment of the lowest lying excited-state, a process which is absent for the BAr F− 4 counter-ion. Taken together, these observations suggest contact-ion pair formation between the cationic metal complex and the PF − 6 anion and, at most, solvent-separated pairing with BAr F− 4 . The dynamic ion pair reorganisation we observe with the PF − 6 counter-ion may substantially modify both the thermodynamic potential available for electron transfer and kinetically inhibit oxidative catalysis, as the anion moves to cover the positively charged end of the molecule, providing a possible mechanistic explanation for recently observed trends in the catalytic activity of these complexes as a function of anion identity in low-polarity solvents. These tunable ion-pair dynamics could prove to be a valuable tool for tailoring the reactivity of both new and extant photocatalysts.

We report evidence of excited-state ion pair reorganisation in a cationic iridium (III) photoredox catalyst in 1,4-dioxane. Microwave-frequency dielectric-loss measurements combined with accurate calculations of dipolar relaxation time allow us to assign both ground and excited-state molecular dipole moments in solution and determine the polarizability volume in the excited-state. These measurements show significant changes in ground-state dipole moment between [Ir[dF(CF3)ppy]2(dtbpy)]PF6 (10.74 Debye) and [Ir[dF(CF3)ppy]2(dtbpy)]BArF4 (4.86 Debye). Photoexcitation of each complex results in population of highly mixed ligand centered and metal-to-ligand charge transfer states with enormous polarizability. Relaxation to the lowest lying excited-state leads to a negative change in the dipole moment for [Ir[dF(CF3)ppy]2(dtbpy)]PF6, and a positive change in dipole moment for [Ir[dF(CF3)ppy]2(dtbpy)]BArF4. These observations are consistent with a sub-nanosecond reorganization with the PF6- counter-ion, which cancels the dipole moment of the lowest lying excited-state, a process which is absent for the BArF-4 counter-ion. Taken together, these observations suggest contact-ion pair formation between the cationic metal complex and the PF6- anion and, at most, solvent-separated pairing with BArF-4. The dynamic ion pair reorganisation we observe with the PF6- counter-ion may substantially modify both the thermodynamic potential available for electron transfer and kinetically inhibit oxidative catalysis, as the anion moves to cover the positively charged end of the molecule, providing a possible mechanistic explanation for recently observed trends in the catalytic activity of these complexes as a function of anion identity in low-polarity solvents. These tunable ion-pair dynamics could prove to be a valuable tool for tailoring the reactivity of both new and extant photocatalysts.

Quantum chemical calculations and laser flash spectroscopy probe spin-dynamics of the photoinduced electron transfer mechanism in two donor–acceptor bisphenanthrolinecopper(i) species, marking the influence of spin on solar energy conversion schemes.